Products for the protection of continuous cast moulds for cast-iron pipes

ABSTRACT

A powder product for the protection of centrifugal casting molds for cast iron pipes includes an inoculating metal alloy and possibly mineral powders, and a strongly reducing metal that is volatile at the temperature of the liquid cast iron. The use of products according to the invention prevents accumulation of dirt on molds and improves the surface condition of the cast iron pipes.

This application is a filing under 35 USC 371 of PCT/FR2003/000181 filedJan. 21, 2003.

FIELD OF THE INVENTION

The invention relates to a powder product designed to protect castingmoulds for cast iron pipes made by centrifugal casting; the castingmoulds used are commonly referred to as “shells”.

STATE OF THE ART

Coatings used for protection of centrifugal casting shells for cast ironpipes were composed firstly of inoculation products and powderrefractory materials, and mixes of silica and bentonite placed byspraying of an aqueous solution. For example, this type of coating isdescribed in Pont-à-Mousson's U.S. Pat. No. 4,058,153.

These products were subsequently replaced by dry spray powders sprayedon the shell before the cast iron was moulded, using the techniquereferred to as “dry spray”. Regardless of the technique used for theirplacement, these products are used to create a thermal barrier effectlimiting the temperature rise of the shell and thus contributing toincreasing its life, and also an inoculating effect on the poured castiron to control the metallurgical structure of the pipe.

It is well known that insufficient inoculation will lead to theformation of carbide in the cast iron, high shrinkage during cooling andfast mould stripping, which helps to increase productivity. But theparts thus obtained will require subsequent heat treatment that may beexpensive.

Depending on the case, it may be preferred to inoculate more to preventthe final heat treatment even though the production rate is lowered, oron the other hand to inoculate only slightly to increase productivityand then apply heat treatment to the cast iron part.

Therefore, the inoculating power of the dry spray may vary within fairlywide limits, however other effects requested from the product aresubjected to more constant requirements.

Therefore, dry spray products are usually composed of a mix of severalcomponents, including an inoculant with a variable efficiency that mayform 30 to 100% of the product, for example a ferro-silicon containing0.1 to 3% of aluminium and calcium and an inert mineral filler, forexample silica or fluorspar that may form between 0 to 70% of theproduct.

Patent FR 2612097 (Foseco) describes the use of FeSiMg type alloys astreatment agents, with particles electrically charged by friction.

These mixes are in the form of powders with size grading always smallerthan 400 μm, but free of fines. For example, a size grading of between50 and 200 μm is well adapted.

PURPOSE OF THE INVENTION

The purpose of the invention is a powder product for the protection ofcentrifugal casting moulds for cast iron pipes, comprising aninoculating metal alloy and possibly inert mineral powders and a highlyreducing metal that is volatile at the temperature of the liquid castiron.

DESCRIPTION OF THE INVENTION

Products according to prior art used as dry spray for the manufacture ofcast iron pipes by centrifugal casting have some disadvantages. Theinert mineral filler added to the mix contributes to increasing therisks of making moulds dirty and the formation of inert mineralinclusions in the cast iron that can appear as surface defects on thepipes.

Furthermore, the applicant has observed that although the addition of astrongly reducing agent such as aluminium protects the shells and theirlife, in some cases, it can increase the risk of occurrence ofunacceptable pitting defects on the surface of the pipes.

Therefore, the applicant's objective was to develop products thatprotect the user from these disadvantages. These products comprise aninoculating alloy, for example based on ferro-silicon, or a mix ofinoculating alloys, possibly a mineral filler and a reducing agent witha content of between 0.3 and 18%, composed of a metal that is volatileat the temperature of the liquid cast iron, that may be a metal fromcolumn 2 in the Mendeleiev classification, and preferably a metal fromcolumn 2a in the periodic table of the elements. The preferred metalsare calcium or magnesium or alloys containing at least one of thesemetals. Silicon alloys are particularly suitable, particularly the CaSialloys. Thus, the following alloy compositions can be used (by weight):Si 58-65%; Ca 27-35%; Fe 2-7%; Al 0.4-2%.

The product preferably contains,

-   -   either between 0.3 and 4% by weight of magnesium, and preferably        between 0.5 and 2%. It is found that the casting mould starts to        get dirty in the form of whitish traces of MgO if the content is        higher than 4%.    -   or 15 to 40% by weight of CaSi alloy, representing a calcium        content of between 4 and 14%.

Tests carried out by the applicant have shown that ferrous alloys of theFeSiCa type, typically containing more than 10% iron and frequentlycalled “CaSiFer” containing (by weight):Si 51-58%; Ca 16-20%; Fe 23-27%; Al 0.3-1.5%

and FeSiMg type alloys containing (by weight):Si 47-53%; Fe 35-48%; Mg 2-12%; Al 0.2-1.5%; Ca 0.1-1.5%, rare earths0-2%,

give disappointing results, well below the results obtained with mixesaccording to the invention.

The corresponding quantities of the different constituents in the finalmix were evaluated as a function of the defects that can arise as aresult of overdoses.

Furthermore, for safety reasons related to preparation of the products,the reducing metals or reducing alloys are not used alone, but rather inthe form of a premix with an inert substance, preferably calciumfluoride, magnesium fluoride or a mix of these two fluorides. Formaximum efficiency, the strongly reducing metal alloy content in thepremix is preferably between 15 and 60%.

The size grading of the products is less than 400 μm, and is preferablyless than 250 μm. Fine particles smaller than 40 μm and preferablysmaller than 50 μm are excluded to prevent dust emission during use.

EXAMPLES Example No. 1

A mix was prepared according to prior art with the followingconstituents:

85% ferro-silicon with 75.2% of Si, 1.3% of Ca and 0.45% of Al, withsize grading of between 50 and 200 μm and 15% of fluorspar with a sizegrading of between 10 and 150 μm.

This product gave satisfactory results when used as a dry spray as areference test; the pipes were stripped after 55 seconds of cooling andthe thickness of ferritic cast iron measured on pipes made in this waywas 35 microns. However, the shells were slightly attacked.

Example No. 2

A mix according to the invention was prepared from the followingconstituents:

55% ferro-silicon with 75.2% of Si, 1.3% of Ca and 0.45% of Al with sizegrading between 50 and 200 μm and 45% of a mix composed of ⅓ fluorsparbetween 10 and 150 μm, and ⅔ of calcium silicide with 60.1% of Si, 31.7%of Ca and 4.3% of Fe.

When used as a dry spray this product gave satisfactory results; thepipes were stripped after 45 seconds cooling and a thickness of 25 μm offerritic cast iron was measured on the pipes made in this way. However,there was no visible attack of the shells.

Therefore, this type of product gives better results than the productmentioned in example No. 1.

Example No. 3

A mix of 50% of magnesium powder with size grading between 50 and 250μm, 25% of magnesium fluoride with size grading between 40 and 250 μm,and 25% fluorspar with size grading between 40 and 250 μm, was prepared.

A mix according to the invention was then prepared consisting of 3% ofthe previous mix and 97% of ferro-silicon with 75.2% of Si, 1.3% of Caand 0.45% of Al with a size grading between 50 and 200 μm.

When used as dry spray test, this product gave results considered to bebetter than those obtained in examples No. 1 and No. 2; the pipes werestripped after 37 seconds cooling and the pipes thus made were found tohave a ferritic cast iron thickness of 30 μm. The surface condition ofthe parts was considered to be excellent.

Example No. 4

A mix according to prior art was prepared with an equivalent compositionof the mix in example 3, composed (by weight) as follows:

-   -   43% of ferro-silicon with 75.2% of Si, 1.3% of Ca and 0.45% of        Al with size grading between 50 and 200 μm and originating from        the same batch as the ferro-silicon used in the previous        example.    -   29.5% of a FeSiMg type alloy with size grading between 50 and        200 μm, analysed to contain 50.7% of Si, 42.0% of iron, 5.2% of        Mg, 1.2% of Ca and 0.35% of Al,    -   26% of metallurgical silicon powder with size grading between 50        and 200 μm containing 98.6% of Si.    -   0.75% of magnesium fluoride with size grading between 40 and 250        μm,    -   0.75% of fluorspar with size grading between 40 and 250 μm.

When used as a dry spray, this product gave results significantly worsethan those obtained in example 3. Pipes were stripped after 50 secondscooling, the observed thickness of ferritic cast iron on the pipes thusmade was 35 μm, and absolutely unacceptable pitting was observed on thesurface of the parts with a density of the order of 25 per m².

1. Powder product for the protection of centrifugal casting molds usedfor the manufacture of cast iron pipes, comprising a mixture of: atleast one inoculating alloy; at least one strongly reducing metal thatis volatile at the temperature of liquid cast iron, the at least onestrongly reducing metal being in the form of at least one non-ferrousalloy, and being present in an amount of 0.3 to 18% by weight of theproduct; and optionally inert mineral powder.
 2. Product according toclaim 1, wherein the at least one inoculating alloy comprises a mixtureof a plurality of inoculating alloys.
 3. Powder product for theprotection of centrifugal casting molds used for the manufacture of castiron pipes, comprising a mixture of: at least one inoculating alloy; atleast one strongly reducing metal that is volatile at the temperature ofliquid cast iron; and optionally inert mineral powder, wherein the atleast one strongly reducing metal comprises magnesium present in anamount of between 0.5 and 2% by weight of the powder product.
 4. Productaccording to claim 1, wherein the at least one non-ferrous alloycontains less than 10% by weight of Fe.
 5. Product according to claim 4,wherein the at least one non-ferrous alloy is an SiCa alloy containing,by weight: Si 58-65%; Ca 27-35%; Fe 2-7%; Al 0.4-2%.
 6. Productaccording to claim 5, containing between 15 and 40% by weight of saidSiCa alloy.
 7. Powder product for the protection of centrifugal castingmolds used for the manufacture of cast iron pipes, comprising a mixtureof: at least one inoculating alloy; at least one strongly reducing metalthat is volatile at the temperature of liquid cast iron; and between0.2and 15% by weight of an inert mineral powder.
 8. Product according toclaim 7, wherein the inert mineral powder is selected from the groupconsisting of calcium fluoride, magnesium fluoride and mixtures thereof.9. Process for manufacturing a powder product for the protection ofcentrifugal casting molds used for the manufacture of cast iron pipes,comprising at least one inoculating alloy, at least one stronglyreducing metal that is volatile at the temperature of liquid cast ironand inert mineral powder, comprising forming a powder premix of the atleast one strongly reducing metal and the inert mineral powder, andmixing the powder premix with the at least one inoculating alloy inpowder form, wherein the at least one strongly reducing metalconstitutes 15 to 60% by weight of the premix.
 10. In a process formolding cast iron comprising bringing molten cast iron into contact withan inside surface of a centrifugal casting mold, the improvementcomprising protecting the mold by applying to the inside surface apowder product comprising at least one inoculating alloy and at leastone strongly reducing metal that is volatile at the temperature of themolten cast iron, before bringing the molten cast iron into contact withthe inside surface of the mold, the at least one strongly reducing metalbeing present in an amount of 0.3 to 18% by weight of the product. 11.Process according to claim 10, wherein the at least one inoculatingalloy comprises a mixture of a plurality of inoculating alloys. 12.Process according to claim 10, wherein the at least one stronglyreducing metal is an element in column II of the periodic table ofelements.
 13. Process according to claim 12, wherein the at least onestrongly reducing metal is an element in subgroup IIa of the periodictable of elements.
 14. Process according to claim 13, wherein the atleast one strongly reducing metal is magnesium or calcium.
 15. Processaccording to claim 14, wherein the product contains between 0.5 and 2%by weight of magnesium.
 16. Process according to claim 10, wherein theproduct additionally comprises between 0.2 and 15% by weight of an inertmineral powder.
 17. Process according to claim 16, wherein the inertmineral powder is selected from the group consisting of calciumfluoride, magnesium fluoride and mixtures thereof.
 18. In a process formolding cast iron comprising bringing molten cast iron into contact withan inside surface of a centrifugal casting mold, the improvementcomprising protecting the mold by applying to the inside surface apowder product comprising at least one inoculating alloy and at leastone strongly reducing metal that is volatile at the temperature of themolten cast iron, before bringing the molten cast iron into contact withthe inside surface of the mold, the at least one strongly reducing metalbeing added in the form of at least one non-ferrous alloy which is anSiCa alloy containing, by weight: Si 58-65%; Ca 27-35%; Fe 2-7%; Al0.4-2%.
 19. Process according to claim 18, wherein the product containsbetween 15 and 40% by weight of said SiCa alloy.
 20. Powder product forthe protection of centrifugal casting molds used for the manufacture ofcast iron pipes, comprising a mixture of: at least one inoculatingalloy; at least one strongly reducing metal that is volatile at thetemperature of liquid cast iron, the at least, one strongly reducingmetal being in the form of at least one non-ferrous alloy; andoptionally inert mineral powder, wherein the at least one non-ferrousalloy is an SiCa alloy containing, by weight: Si 58-65%; Ca 27-35%; Fe2-7%; Al 0.4-2%.
 21. Product according to claim 20, containing between15 and 40% by weight of said SiCa alloy.